This invention relates to a method and an apparatus for fabricating a thin film semiconductor device, which may be suitably used for fabricating, for example, a thin film transistor (TFT).
In recent years, an attention is paid to TFT using a polycrystalline silicon (Si) film (hereinafter called a polycrystalline Si TFT) as a pixel switching device in an active matrix type liquid crystal display, etc. Polycrystalline Si TFTs, in most cases, have a so-called top gate structure whose gate electrode is located above its active layer as viewed from the substrate.
A typical method for fabricating the polycrystalline Si TFT having the top gate structure first produces on a substrate a hydrogenated amorphous Si thin film (hereinafter called an a-Si:H thin film) by a plasma CVD process, then crystallizes the thin film into a polycrystalline Si thin film in vacuum by using thermal or light energy, and then produces a gate insulating film on the polycrystalline Si thin film. Formation of the gate insulating film has so far been done after changing the a-Si:H thin film into the polycrystalline Si thin film and then breaking the vacuum of the processing chamber (see, for example, Extended Abstract on SSDM, 967(1990) and Report of Group on Application of Electronic Properties of Matter, JSAP Catalog No. AP902204 No. 432, 19(1990)).
Also known and widely used is a plasma hydrogenation process as a method for reducing defects of crystalline thin films (IEEE, EDL. vol. 10 (1989)123).
However, once the vacuum is broken after producing the polycrystalline Si thin film, the clean surface of the polycrystalline Si thin film tends to be contaminated by molecules of water and hydrocarbon compounds, or changes in nature due to reaction with gases (oxidation, etc). If the gate insulating film is formed on such a contaminated or changed surface of the polycrystalline Si thin film, adverse influence occurs to the performance (on current, threshold voltage, subthreshold swing factor, etc.), reliability (breakdown voltage, etc.) and uniformity (in-wafer characteristics) of a transistor fabricated.
It is also reported that the conventional plasma hydrogenation process for reducing defects of a crystalline thin film may cause large changes in electrical conductivity of the polycrystalline thin film if an SiO.sub.2 film exists thereon, in particular, when the crystalline film is a polycrystalline Si thin film (Mat. Res. Symp. Proc. E5,4(1992)). That is, since the conventional plasma hydrogenation process once takes out the substrate in the outside air for this process after the polycrystalline Si thin film is formed on the substrate by the laser crystallization method, the surface of the polycrystalline Si thin film is exposed to the outside air so that a native oxide film is formed thereon. If, in this state, plasma hydrogenation of the polycrystalline Si thin film is performed, electrical conductivity of the polycrystalline Si thin film also changes by several orders of magnitude as shown in FIG. 1. This large change in electrical conductivity causes variance in characteristics of thin film semiconductor devices, and is therefore a matter to be considered.